This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
It is well known in the art to use so-called Conditional Access (CA) technology to protect digital content, such as e.g. premium value digital television programs. Such technology uses different cryptographic protocols to encrypt and later decrypt the digital content, and also to protect the terminals from unauthorized use.
The CA technology is implemented in receivers, for example television sets and so-called Set-Top Boxes (STBs), using some kind of (usually removable) security element that has been provided, directly or indirectly, to a user by a CA provider. A preferred implementation of such a security element is a smartcard, which is a well-known in the art.
It will be appreciated that it is usually necessary to pay in order to access CA protected digital content and that it is quite common for pirates to try to ‘hack’ the CA system in order to access the CA protected digital content without having to pay for it. As smartcards become more and more secure—i.e. harder and harder to hack—pirates tend to attack the receivers directly.
Naturally, receiver manufacturers have worked in improving the security of their receivers by implementing certain security features required and specified by the supported CA provider, and using a CA dedicated version of the main IC which implements in particular:                a Secret Root Key (SRK), which can be implemented as a 128-bit symmetric key, and        a Public Authentication Key (PUAK), which advantageously can be implemented as an asymmetric 1024 or 2048 bit RSA key.        
These security features that are implemented by chip providers must satisfy the security requirements of a particular CA provided. It is quite likely that a first CA provider A has different security requirements that a second CA provider B. This can mean that components that are customized for A may not work for or be acceptable to B; besides, CA providers usually do not communicate their solutions to other CA providers.
Further, as CA providers often are commercial rivals, it may be difficult for them to agree on a common set of security requirements. This commercial rivalry thus pushes CA providers to improve their security requirements and to keep them secret.
For this reason, certain CA provides provide a proprietary security block for the chip provider to use instead of the chip providers ‘standard’ security features.
It can thus be seen that different CA providers provide different security features.
It will also be appreciate that the clients of the CA providers, such as pay TV operators, have an interest in being able to use products from a plurality of CA providers. So far, however, once an operator has selected a CA provider, it is difficult to switch to a new one for the following reasons:                it is costly to change the installed receivers; this should be done to ensure that both old and new receivers can work the same way,        it is difficult to provide to the new CA provider a platform that enables the required security features without risking decreased security.        
For this reason, pay TV operators would like to be able to be as independent as possible from the CA providers so that the same receiver hardware can be used with different CA systems so that the different security requirements are independently supported.
The requirement that a receiver be compatible with a plurality of CA providers can be met if the security functions, i.e. the security block, of each CA provider is integrated into a chip provided by a chip manufacturer.
SW Authentication Chain:
In order to ensure that only authorized software is installed on a receiver, e.g. by verification at power up that the receiver embedded software (or a second stage authentication key) is valid, it is known to digitally sign the software (or the second stage authentication key) using a Private Asymmetric Key (PRAK) that advantageously is held by the CA provider. At power up, the receiver uses its Public Authentication Key (PUAK), which corresponds to the PRAK, to verify the signature of the receiver embedded software (or the second stage authentication key), as is well known in the art.
The PUAK for a CA provider is part of the customization of a chip to meet the security requirements of the CA provider.
So far, the PUAK of a single CA provider is stored in the chip. This means that it is not possible for the chip to be compatible with a plurality of CA systems, even if all the other security features of another CA provider are implemented in the chip.
The skilled person will thus appreciate that there is a need for a solution that allows a receiver to store and use a plurality of PUAKs. This invention provides such a solution.